Tree dwarfish but thrifty, attaining a height of twenty to twenty-five feet; trunk reaches eight inches in diameter and bears its head rather low, three to five feet from the ground; bark gray with a tinge of red, smooth, with transverse cracks; branches upright-spreading, rigid, compact, short-jointed, and more or less thorny; branchlets pubescent, slender, reddish-brown or drab.
Winter-buds small, conical, pointed or obtuse, free or appressed; leaves small, ovate or obovate; apex obtuse or abruptly pointed, base cuneate or narrowed and rounded, margins finely and closely, sometimes doubly serrate or crenate, usually glandular; texture thin and firm; upper surface slightly rugose, dark green, slightly hairy; lower surface paler and soft, pubescent; petioles one-half inch long, slender, pubescent, tinged with red; glands few or glandless.
Flowers expand with or after the leaves, one inch or less in size; borne variously but usually in lateral, umbel-like clusters, one, two or rarely three from a bud, on slender pedicels, which are pubescent and one-half inch in length; calyx-tube campanulate, glabrous or nearly so, green or tinged with red; calyx-lobes narrow, obtuse or acute, glandular-serrate, glabrous or pubescent, reflexed; petals white or creamy in the bud, broadly oval, entire or dentate, reflexed, claw short; stamens about twenty-five, as long as the petals; anthers yellow, often tinged with red; pistil glabrous and nearly as long as the stamens.
Fruit ripens from early to late; globular or oval, often necked, less than an inch in diameter, variously colored but usually bluish-black or amber-yellow, with a heavy bloom; skin thin, tough; stem slender, one-half inch long, more or less pubescent; cavity shallow, narrow; apex roundish or flattened; suture indistinct or a line; flesh firm, yellow, juicy, sweet or acid; stone clinging or free, somewhat turgid, ovoid, nearly smooth, ridged on one edge and grooved on the other.
There is a great diversity of opinion among botanists as to what Linnaeus meant to include in his Prunus insititia. His description of the species is not definite and can be made to apply to any one of several very distinct plums. But the botanists who recognize the species usually include in it, among cultivated plums, the Bullaces and the Damsons, plums which differ only in the shape of the fruit, the former being round and the latter oval. Some of the texts noted in the references for this species also place the St. Julien and the Mirabelle plums here. In The Plums of New York the authors consider the Bullaces, Damsons, the St. Julien and the Mirabelles as belonging to this species.
It is true that Linnaeus established at an earlier date than the naming of Prunus insititia his Prunus domestica damascena, in which the varietal name indicates that he meant the Damsons, but the description of the variety taken by him from Bauhin’s Pinax[60] making the plum large, sweet and dark purplish, cannot be made to apply to this fruit, nor can it be connected definitely with any other plum; this being true, and since Linnaeus refers to no type specimen, figure, or locality, his Prunus domestica damascena according to current botanical practices in America, should be rejected.
The trees of the Insititia varieties are readily distinguished from the Domestica sorts in having a dwarfer and more compact habit; much smaller and more ovate leaves with more closely serrate margins; branches more finely divided, more slender, with shorter joints, and bearing spines or spinescent spurs; having a more abundant and a more clustered inflorescence, with smaller flowers, a glabrous instead of a pubescent pistil and calyx-tube; reflexed calyx-lobes where in Domestica they are often erect; and flowers appearing nearly a week later. The number of stamens in Prunus domestica averages about thirty; in Prunus insititia, about twenty-five. The fruit-characters of Prunus insititia are even more distinctive. The fruits are smaller, being less than an inch in diameter, more nearly round or oval, more uniform in shape, never strongly compressed as in Domestica, with a less distinct suture and more often with a pronounced neck. The color is usually the Damson purple or the Mirabelle yellow, with no intermediate colors as in Domestica and with few or but slight variations as compared with the other species. The plums are sweet or sour with a very much smaller range in flavor in the case of the Insititias and withal very distinct from that of Prunus domestica. The stones are smaller, more oval and much more swollen.
In variability the Insititia plums are quite the reverse of the Domesticas, almost wholly lacking this quality. These plums have been cultivated over two thousand years, yet there is seemingly little difference between the sorts described by the Greeks and Romans at the beginning of the Christian Era and those we are now growing. So, too, one often finds half-wild chance seedlings with fruit indistinguishable from varieties under the highest cultivation. This pronounced immutability of the species is one of its chief characteristics.
There are probably several sub-divisions of Prunus insititia but material does not exist in America for the proper determination of the true place for these forms, and the Old World botanists cannot agree in regard to them. It is probable that Prunus subsylvestris Boutigny[61] and Prunus pomarium Boutigny[62] belong to Prunus insititia and almost beyond question Prunus syriaca Koehne[63] is the yellow-fruited Mirabelle of this species. Prunus insititia glaberrima Wirtg,[64] occasionally found in the herbaria of Europe has, with its small, roundish-obovate leaves, but little appearance of Prunus insititia and may be, as Schneider surmises,[65] a cross between Prunus spinosa and the Myrobalan of Prunus cerasifera.
The Insititia plums are second in importance only to the Domesticas. Their recorded history is older. This is the plum of the Greek poets, Archilochus and Hippona, in the Sixth Century B. C.[66] Theophrastus, the philosopher, mentioned it three hundred years before Christ, as did Pollux, the writer and grammarian, a century before the Savior, while Dioscorides, the founder of botany, during the last named period, distinguishes between this plum and one from Syria, presumably a Domestica. This is one of the twelve kinds of plums described by Pliny (see page 17) who calls it the Damascene, so-called from Damascus in Syria, and says of it, “introduced long since into Italy.” It is the Damask plum of Columella when in his tenth book he says: